Naturally occurring deposits containing oil and natural gas have been located throughout the world. Given the porous and permeable nature of the subterranean structure, it is possible to bore into the earth and set up a well where oil and natural gas are pumped out of the deposit. These wells are large, costly structures that are typically fixed at one location. As is often the case, a well may initially be very productive, with the oil and natural gas being pumpable with relative ease. As the oil or natural gas near the well bore is removed from the deposit, other oil and natural gas may flow to the area near the well bore so that it may be pumped as well. However, as a well ages, and sometimes merely as a consequence of the subterranean geology surrounding the well bore, the more remote oil and natural gas may have difficulty flowing to the well bore, thereby reducing the productivity of the well.
To address this problem and to increase the flow of oil and natural gas to the well bore, companies have employed the well-known technique of fracturing the subterranean area around the well to create more paths for the oil and natural gas to flow toward the well. As described in more detail in the literature, this fracturing is accomplished by hydraulically injecting a fluid at very high pressure into the area surrounding the well bore. This fluid must then be removed from the fracture to the extent possible to ensure that it does not impede the flow of oil or natural gas back to the well bore. Once the fluid is removed, the fractures have a tendency to collapse due to the high compaction pressures experienced at well-depths, which can be more than 20,000 feet. To prevent the fractures from closing, it is well-known to include a propping agent, also known as a proppant, in the fracturing fluid. The goal is to be able to remove as much of the injection fluid as possible while leaving the proppant behind to keep the fractures open. As used in this application, the term “proppant” refers to any non-liquid material that is present in a proppant pack and provides structural support in a propped fracture. “Anti-flowback additive” refers to any material that is present in a proppant pack and reduces the flowback of proppant particles but still allows for production of oil at sufficient rates. The terms “proppant” and “anti-flowback additive” are not necessarily mutually exclusive, so a single particle type may meet both definitions. For example, a particle may provide structural support in a fracture and it may also be shaped to have anti-flowback properties, allowing it to meet both definitions.
Several properties affect the desirability of a proppant. For example, for use in deep wells or wells whose formation forces are high, proppants must be capable of withstanding high compressive forces, often greater than 10,000 pounds per square inch (“psi”). Proppants able to withstand these forces (e.g., up to and greater than 10,000 psi) are referred to as high strength proppants. If forces in a fracture are too high for a given proppant, the proppant will crush and collapse, and then no longer have a sufficient permeability to allow the proper flow of oil or natural gas. Other applications, such as for use in shallower wells, do not demand the same strength proppant, allowing intermediate strength proppants to suffice. These intermediate strength proppants are typically used where the compressive forces are between 5,000 and 10,000 psi. Still other proppants can be used for applications where the compressive forces are low. For example, sand is often used as a proppant at low compressive forces.
In addition to the strength of the proppant, one must consider how the proppant will pack with other proppant particles and the surrounding geological features, as the nature of the packing can impact the flow of the oil and natural gas through the fractures. For example, if the proppant particles become too tightly packed, they may actually inhibit the flow of the oil or natural gas rather than increase it.
The nature of the packing also has an effect on the overall turbulence generated through the fractures. Too much turbulence can increase the flowback of the proppant particles from the fractures toward the well bore. This may undesirably decrease the flow of oil and natural gas, contaminate the well, cause abrasion to the equipment in the well, and increase the production cost as the proppants that flow back toward the well must be removed from the oil and gas.
The useful life of the well may also be shortened if the proppant particles break down. For this reason, proppants have conventionally been designed to minimize breaking. For example, U.S. Pat. No. 3,497,008 to Graham et al. discloses a preferred proppant composition of a hard glass that has decreased surface flaws to prevent failure at those flaws. It also discloses that the hard glass should have a good resistance to impact abrasion, which serves to prevent surface flaws from occurring in the first place. These features have conventionally been deemed necessary to avoid breaking, which creates undesirable fines within the fracture.
The shape of the proppant has a significant impact on how it packs with other proppant particles and the surrounding area. Thus, the shape of the proppant can significantly alter the permeability and conductivity of a proppant pack in a fracture. Different shapes of the same material offer different strengths and resistance to closure stress. It is desirable to engineer the shape of the proppant to provide high strength and a packing tendency that will increase the flow of oil or natural gas. The optimum shape may differ for different depths, closure stresses, geologies of the surrounding earth, and materials to be extracted.
The conventional wisdom in the industry is that spherical pellets of uniform size are the most effective proppant body shape to maximize the permeability of the fracture. See, e.g., U.S. Pat. No. 6,753,299 to Lunghofer et al. Indeed, the American Petroleum Institute's (“API's”) description of the proppant qualification process has a section dedicated to the evaluation of roundness and sphericity as measured on the Krumbein scale. However, other shapes have been suggested in the art. For example, previously-mentioned U.S. Pat. No. 3,497,008 to Graham et al. discloses the use of “particles having linear, parallel, opposite surface elements including cylinders, rods, paralellepipeds, prisms, cubes, plates, and various other solids of both regular and irregular configurations.” (Col. 3, lines 34-37.) According to that patent, the disclosed shape configuration has several advantages when used as a proppant, including increased conductivity over spherical particles (col. 4, lines 29-35), greater load bearing capacity for the same diameter as a spherical particle (col. 4, lines 36-38), a higher resistance to being embedded in the fracture wall (col. 4, lines 45-47), and a lower settling rate (col. 4, lines 58-60).
Despite this disclosure of the potential advantages of using rod-like particles for proppants, the industry had not embraced the suggestion. The applicants are not aware of any rod-like particles on the market that are used as proppants or anti-flowback additives. Indeed, more recent patents cast doubt on the effectiveness of using rod-like shapes. For example, U.S. Pat. No. 6,059,034 to Rickards et al. discloses the mixing of rod-like fibrous materials with another proppant material to prevent proppant movement and flowback. According to that patent, “in practice this method has proven to have several drawbacks, including reduction in fracture conductivity at effective concentrations of fibrous materials, and an effective life of only about two years due to slight solubility of commonly used fiber materials in brine. In addition, fiber proppant material used in the technique may be incompatible with some common well-treating acids, such as hydrofluoric acid.” (Col. 2, lines 36-43.) Although the rod-like fibrous materials are used in conjunction with another proppant, the patent suggests that rod-like particles in a fracturing fluid are undesirable.
Another property that impacts a proppant's utility is how quickly it settles both in the injection fluid and once it is in the fracture. A proppant that quickly settles may not reach the desired propping location in the fracture, resulting in a low level of proppants in the desired fracture locations, such as high or deep enough in the fracture to maximize the presence of the proppant in the pay zone (i.e., the zone in which oil or natural gas flows back to the well). This can cause reduced efficacy of the fracturing operation. Ideally, a proppant disperses equally throughout all portions of the fracture. Gravity works against this ideal, pulling particles toward the bottom of the fracture. However, proppants with properly engineered densities and shapes may be slow to settle, thereby increasing the functional propped area of the fracture. How quickly a proppant settles is determined in large part by its specific gravity. Engineering the specific gravity of the proppant for various applications is desirable because an optimized specific gravity allows a proppant user to better place the proppant within the fracture.
Yet another attribute to consider in designing a proppant is its acid-tolerance, as acids are often used in oil and natural gas wells and may undesirably alter the properties of the proppant. For example, hydrofluoric acid is commonly used to treat oil wells, making a proppant's resistance to that acid of high importance.
Still another property to consider for a proppant is its surface texture. A surface texture that enhances, or at least does not inhibit, the conductivity of the oil or gas through the fractures is desirable. Smoother surfaces offer certain advantages over rough surfaces, such as reduced tool wear and a better conductivity, but porous surfaces may still be desirable for some applications where a reduced density may be useful.
All of these properties, some of which can at times conflict with each other, must be weighed in determining the right proppant for a particular situation. Because stimulation of a well through fracturing is by far the most expensive operation over the life of the well, one must also consider the economics. Proppants are typically used in large quantities, making them a large part of the cost.
Attempts have been made to optimize proppants and methods of using them. Suggested materials for proppants include sand, glass beads, ceramic pellets, and portions of walnuts. The preferred material disclosed in previously-mentioned U.S. Pat. No. 3,497,008 is a hard glass, but it also mentions that sintered alumina, steatite, and mullite could be used. Conventional belief is that alumina adds strength to a proppant, so many early proppants were made of high-alumina materials, such as bauxite. The strength of these high-alumina materials is believed to be due to the mechanical properties of the dense ceramic materials therein. See, e.g., U.S. Pat. Nos. 4,068,718 and 4,427,068, both of which disclose proppants made with bauxite.
Bauxite is a natural mineral comprising various amounts of four primary oxides: alumina (Al2O3, typically from about 80% to about 90% by weight), silica (SiO2, typically from about 1% to about 12% by weight), iron oxide (Fe2O3, typically from about 1% to about 15% by weight), and titania (TiO2, typically from about 1% to about 5% by weight). After calcining or sintering, bauxite is known to have a higher toughness but a lower hardness than technical grade alumina-based ceramics. Since toughness is a primary mechanical characteristic to consider in improving the crush resistance or compressive strength of ceramics, bauxite is of interest for use in proppants. The microstructure of bauxite is characterized primarily by three phases: 1) a matrix of fine alumina crystal; 2) a titania phase where titania is complexed with alumina to form aluminum titanate (Al2TiO5); and 3) a mullite phase (3Al2O3, 2SiO2). For the first two phases a partial substitution of aluminum by iron atoms is possible. To achieve good mechanical characteristics as a proppant, bauxite with lower levels of silica and iron oxide are preferred.
For example, previously-mentioned U.S. Pat. No. 4,427,068 discloses a spherical proppant comprising a clay containing silica that adds a glassy phase to the proppant, thereby weakening the proppant. Furthermore, the silica of that patent is so-called “free” silica. In general, high amounts of silica reduce the strength of the final proppant. In particular, it is believed that proppants containing more than 2% silica by weight will have reduced strength over those with lower silica contents. Other so-called impurities are also believed to reduce the strength of the proppant.
Early high strength proppants were made using tabular alumina which was a relatively expensive component. For this reason, the industry shifted from using tabular alumina to other alumina sources, such as bauxite. By the late 1970's, the development focus in the industry shifted from high strength proppants to intermediate or lower strength, lower density proppants that were easier to transport and use, and were less expensive. Over the next 20 years, the industry focused on commercialization of lower density proppants and they became commonly used. The primary method of reducing the density of proppants is to replace at least a portion of the higher density alumina with lower density silica. According to U.S. Pat. No. 6,753,299, “the original bauxite based proppants of the early 1970's contained >80% alumina (Cooke). Subsequent generations of proppants contained an alumina content of >70% (Fitzgibbons), 40% to 60% (Lunghofer), and later 30% to <40% (Rumpf, Fitzgibbons).” Thus, as to both product development and proppant use, there was a retreat in the industry from proppants manufactured from high-alumina materials such as bauxite.
Today, as resources become more scarce, the search for oil and gas involves penetration into deeper geological formations, and the recovery of the raw materials becomes increasingly difficult. Therefore, there is a need for proppants that have an excellent conductivity and permeability even under extreme conditions. There is also need for improved anti-flowback additives that will reduce the cost of production and increase the useful life of the well.